Starshun

And this is a great article on the problems of designing high altitude airships

People were designing it for mean ambient wind speeds. It turned out, you have to design them not for the mean, but to account the extremes of wind speed over a long period of time. If you want to be [there] up for a year, you need design for extremes you see in a year. The extremes at altitude can be three to four times greater than the mean. The required power at stationkeep scales with the cube of the velocity. Double your wind speed and you increase your power requirements by a factor of eight.